Warm, light, cheap, pick two. That's how it works. There is nothing inherently wrong with a synthetic fill bag and in some circumstances, they are better than down. If I was kayaking or canoeing in marginal conditions, a synthetic fill bag would be my choice. I have a TNF Cat's Meow, a bag rated at 20F. Mine is old (25 years or so) so I doubt seriously it would be warm at 20F and if I recall right, it wasn't warm at 20F when new. But, for what it is supposed to be-a relatively warm bag at a decent price, it is a good buy. On the other hand, my down bag of similar vintage is good to around 20F, which is close to its rating (-5C, which is +23F), but cost twice as much.